Thin film shape memory alloy actuated flow controller

ABSTRACT

A flow controller for use in microelectromechanical systems. The principal components of the controlled are a microvalve and sensor which are micromachined on one surface of a substrate that is formed with a fluid flow channel. The microvalve includes a shape memory alloy actuator element that is operated by a feedback signal from a control circuit. The sensor can be a fluid flow rate sensor or a fluid temperature sensor or a fluid pressure sensor. Conditions in the channel are sensed for generating the feedback signal.

CROSS-REFERENCE TO PRIOR APPLICATION

[0001] This application claims the benefit under 35 USC §119(e) of U.S.provisional application serial No. 60/273,621 filed Mar. 7, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to microelectromechanical (MEMS) sytems,and more particularly to flow controllers for use in MEMS systems.

[0004] 2. Description of the Related Art

[0005] Most flow controllers are made of discrete components and aresingle channel. This invention has fabricated multi-channel flowcontrollers integrated onto a single substrate using microfabrication(MEMS) processes. This approach integrates the normally separatedfunctions of flow measurement, feedback, and control enablingminiaturization of these devices. The functions of sensing, feedbackcontrol, and communication with a host computer are performed by adedicated microprocessor on the same substrate. Miniaturization ofcomponents allows for the reduction of dead volume and increasedportability.

[0006] Miniaturization of components also allows for the incorporationof multiple channels of various sizes within the footprint of aconventional single channel flow controller made from discretecomponents. A multi-channel flow controller enables changes in flowranges without having to change out the flow controller itself as is nowthe practice in the semiconductor industry. This saves on labor costsand reduces the number of different ranges of flow controllers that mustbe stocked.

[0007] Flow controllers require a valve. In the prior art, the lack ofsuitably fast, reliable valves produced by MEMS processes has preventedthe fabrication of truly integrated MEMS-produced flow controllers.

OBJECTS AND SUMMARY OF THE INVENTION

[0008] It is a general object of the invention to provide new andimproved fluid flow controllers which are of sufficient minature sizefor enabling their use in MEMS applications.

[0009] This invention provides minature proportional microvalves thatcan be configured in multiple valve arrays. These microvalves are muchsmaller than the smallest currently available solenoid valve. It is theuse of this very small valve along with thin film sensors, which sensefluid flow and/or pressure and/or temperature, that enables asignificant reduction in size.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a top plan view of a multiple valve array in accordancewith one embodiment of the invention.

[0011]FIG. 2-A is an exploded isometric front view of an individualmicrovalve used in the array of FIG. 1.

[0012]FIG. 2-B is an exploded isometric rear view of the microvalve ofFIG. 2-B.

[0013]FIG. 3 is a top plan view showing two types of ceramic valvesubstrates for use in the microvalve of FIGS. 2-A and 2-B.

[0014]FIG. 4 is a block diagram showing the overall control circuit usedin the invention.

[0015]FIG. 5 is a circuit diagram for the first pressure sensor circuitin the invention.

[0016]FIG. 6 is a circuit diagram for the second pressure sensor circuitin the invention.

[0017]FIG. 7 is a circuit diagram for the temperature sensor circuit inthe invention.

[0018]FIG. 8 is a circuit diagram for the flow sensor circuit in theinvention.

[0019]FIG. 9 is a circuit diagram for the microprocessor circuit in theinvention.

[0020]FIG. 10-A is a front view and left side view of a manifold for usein a dual range flow controller embodiment of the invention.

[0021]FIG. 10-B is a back view and right side view of the manifold ofFIG. 10-A.

[0022]FIG. 11 is a top plan view of a dual channel flow controllerincorporating the manifold of FIGS. 10-A and 10-B.

[0023]FIG. 12-A is a schematic plan view of a flow sensor in theinvention.

[0024]FIG. 12-B is a schematic cross sectional view of the flow sensorof FIG. 12-A.

[0025]FIG. 12-C is a schematic isometric view of the flow sensor of FIG.12-A.

[0026]FIG. 13 is a block diagram of a digital mass flow controller usinga flow restrictor for use in the invention.

[0027]FIG. 14-A is a block diagram showing a mass flow controller thatis responsive to a flow sensor that measures differential pressure.

[0028]FIG. 14-B is a block diagram showing a mass flow controller thatis responsive to a flow sensor that measures absolute pressure.

[0029]FIG. 15 is a simplified flow chart of a flow control algorithm forthe mass flow controller of FIG. 13.

[0030]FIG. 16 is a block diagram showing a dual range mass flowcontroller in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] In accordance with one preferred embodiment, the inventionprovides a flow controller 13 (FIG. 11) of minature size suitable foruse in microelectromechanical (“MEMS”) systems that controls fluid flow.

[0032] Flow controller 13 includes a thin film microvalve array, showngenerally at 10 in FIG. 1, which is formed by micromaching on a chipsubstrate. Any desired number of microvalves can be employed in such anarray, and in the illustrated embodiment the array comprises fourmicrovalves 12, 12′ 12″ and 12′″. Each microvalve is comprised of avalve actuator 15 (shown only for valve 12) having a thin film operatingelement 17 formed of a shape memory alloy (SMA) to control fluid flow, asensor or sensors (FIGS. 12-A, 12-B and 12-C) to measure flow rateand/or fluid temperature and/or fluid pressure, and channels 19, 21(FIG. 11) connecting the sensor(s) and microvalve through which flowtakes place.

[0033] SMA operating element 17 in actuator die 22 (FIGS. 2-A and 2-B)preferably is comprised of nickel-titanium thin film shape memory alloy.The alloy undergoes a crystalline phase change transformation, andresulting shape change, from a low temperature deformable shape to ahigh temperature memory shape when the element is heated through thealloy's phase change transformation temperature. When so heated, theshape change can be used to perform work. In this invention the workoccurs as the operating element changes shape so as to move within oracross a valve channel to partially or completely open or close thefluid flow path. The actuator is proportional in operation. The thinfilm SMA actuator can be formed on a substrate by the methods set forthin U.S. Pat. No. 5,061,914 to Busch et. al., the disclosure of which isincorporated by this reference.

[0034] The assembly of components of microvalve 12, which is typical inthe invention, is shown in the exploded views of FIGS. 2-B and 2-B. Themicrovalve 12 is comprised of a housing 14, O-ring 16, orifice die 18,spacer 20, actuator die 22, bias spring 24 and cover 26. Thesecomponents are assembled together as shown in the Figures by suitablemeans such as pogo pins 28 and screws 30. Bias spring 24 serves thefunction of applying a yieldable force against actuator element 15 so asto move it from the memory shape to the deformable shape when theelement's temperature is below the transformation temperature. The forceof element 15 when actuated toward its memory shape is sufficent toovercome the spring bias force.

[0035] The SMA operated microvalves of the invention can provideproportional control, which is desirable for many applications. The flowsensor measures fluid mass passing by a point in a fixed time, ormeasures the pressure drop across a calibrated orifice. All of thecomponents can be fabricated on a single small chip by known methods ofphotolithography and micromaching of silicon substrates.

[0036] Particular advantages of the invention are that the SMA actuatorsare so small that multiple actuators may be placed in a space withoutincreasing the package size. This has the important consequence that ifseveral valves are packaged together with each having its own flowchannel, then the dynamic range of the flow controller can be extended.Conventional sensors always have a range over. which they work best. Asensor that measures accurately to 0.1% in the range of 10 to 1000ml/min is not accurate in the range of 0.001 to 1 ml/min. In theinvention when a set-point is specified, the appropriate range isselected by software. An advantage is that the user need only buy one,not multiple, controllers. Then when flow rate must be changed, it canbe done in the system software without opening a line and interruptingthe process.

[0037]FIG. 3 shows a substrate 32 of a ceramic with wire-bondedelectrical leads 34. Mounted on the substrate are two or moremicrovalves 12, 12′ of the valve array, a sensor 36, which can compriseeither a flow, pressure or temperature sensor, and an electroniccontroller board 38. The microvalves can be fabricated in accordancewith the methods of U.S. Pat. No. 5,325,880 to A. David Johnson et. al.,the disclosure of which is incorporated by this reference. The channelsfor providing fluid flow paths between the valves, sensors andinlet/outlet ports are formed in layers (not shown) of a suitablematerial that can be sputter deposited over the substrate.

[0038] The overall control circuit shown in FIG. 4 provides feedback toadjust each microvalve so that the measured flow matches a preset value,called the set-point. Feedback can be performed by a microprocessor 60running on suitably programmed software. The circuit comprises firstpressure sensor circuit 52, second pressure sensor circuit 54,temperature sensor circuit 56, flow sensor circuit 58 and microprocessorcircuit 60.

[0039] The first and second pressure sensor circuits 52 and 54 are shownin detail in FIGS. 5 and 6, respectively. Temperature sensor circuit 56is shown in detail in FIG. 7, and flow sensor circuit 58 is shown indetail in FIG. 8. Microprocessor circuit 60 is shown in detail in FIG.9. This circuit 60 comprises a suitable programmable microprocessor suchas a PIC16C74A chip 61. First and second pressure sensor circuits 52 and54 have respective leads 64 and 66 which connect with respective chipterminals 68 and 70. Temperature sensor circuit 56 has a lead 72 whichconnects with chip terminal 74. And flow sensor circuit 58 has a lead 76which connects with chip terminal 78.

[0040] A suitable MEMS flow sensor 36 is provided on at least one of thesubstrates to control flow rates. For this purpose, the microprocessorcircuit 60 of FIG. 9 is programmed with suitable software. A feedbackloop is incorporated in the software to minimize fluctuations. In thepreferred embodiment the microprocessor chip is programmed to controlthe pulse width to the valve actuator via RS232 serial communication.

[0041] The invention employs a graphical user interface (GUI) which isprogrammed in a suitable language such as Visual Basic ™. For purposesof illustration, dual channel operation has been chosen. Multi-channeloperation can be achieved by expanding the GUI interface and increasingthe number of channels on the controller hardware. The GUI interface canbe displayed on a PC and is connected to the PIC16C74A via RS232communication. The operator chooses a sensor supported by the softwareand selects the desired communications port. Then the operator selectsthe desired channel. The operator then enters the setpoint for eachchannel.

[0042] FIGS. 10-A and 10-B show a manifold 80 for a dual channel flowcontroller embodiment of the invention. FIG. 11 shows the manifold 80with flow restrictors 82 and 84 extending from opposite sides. Thismanifold enables a user to switch between various channel sizes withouthaving to switch out the controller itself. The flow controllercomprises first and second ceramic substrates mounted on the Delryn ®plastic manifold 80, with flow range selection achieved by the flowrestrictors. The first substrate comprises two differential pressuresensors of the type which measure differential pressure drop across therestricor tubes. The flow restrictors 82 and 84 are formed of stainlesssteel tubing which are pinched at 85 and 87 to smaller effective crosssections while connecting to a flow meter. This flow controller has tworanges—from 1 to 100 SCCM and from 1 to 1,000 SCCM. The controllerrequires two valves for shut-off and two proportional valves. It alsocomprises a four-valve multiple microvalve array.

[0043] As shown in FIG. 11, manifold 80 is formed with a pair ofmicrovalves 89, 91 which are connected through channels 21, 21′ withrespective flow restrictors 82, 84. Inlet ports 93, 95 and outlet ports97, 99 direct flow to and from the respective microvalves. Pressuretransducers 101 and 103 are provided in the flow paths for therespective microvalves.

[0044] Components of sensor 86 are shown in schematically in the planview of FIG. 12-A, the cross section view of FIG. 12-B and isometricview of FIG. 12-C. This sensor is comprised of three resistors H, T1 andT2 located at the middle of a layer of Si membrane formed byanisotroopic wet etching of the Si layer.

[0045] The three resistors are formed by diffusing boron into the Simembrane. To configure the flow sensor, the resistor H is used as theheater element and resistors T1 and T2 are used as temperature sensors,with one temperature sensor located upstream related to the heater andthe other downstream, as shown for T1 in FIG. 12-A where the flow isdepicted as from right to left.

[0046] When there is no fluid flow, the heat produced by the heater H isequally distributed to T1 and T2. When there is flow, there is animbalance in the heat distribution which is detected by the circuitmeasuring the differential resistance of T1 and T2.

[0047]FIG. 13 is a block diagram showing a mass flow controller systemfor operating an SMA microvalve actuator 79 in the invention. The flowfrom the source is read by flow sensor 81 and converted from analog todigital signals by the microprocessor circuit. At step 83 the digitalsignal is compared to the set-point that has been specified by the user.The average current into valve 79 is then increased or decreased,thereby regulating the downstream flow. All flow sensor, valve andelectronics components are micrfrabricated and attached to a commonsubstrate.

[0048] Flow sensor 81 can be of the type shown in the block diagram ofFIG. 14-A which measures pressure differential across a restrictor, asin the embodiment explained in connection with FIG. 11. Alternatively,the flow sensor could be of the type shown in the block diagram of FIG.14-B which is responsive to measurement of absolute pressure. In thiscase, the software would be altered to provide feedback as a pressureregulator. In both systems of FIGS. 14-A and 14-B, signals from thepressure sensor are compared with the set-point value to control aproportional valve by means of the microprocessor software.

[0049]FIG. 15 is a simplified flow chart of the flow control algorithmfor the mass flow controller system of FIG. 13. Software resident inmicroprocessor 60 of FIG. 4 controls the average current to microvalveactuator 79 (FIG. 13) to bring measured flow into equality with theset-point flow. All functions of analog to digital conversion, timing,comparison of measured flow to set-point and communication with the hostcomputer are embodied in the single chip.

[0050]FIG. 16 shows a block diagram for another embodiment providing adual range mass flow controller. Conventional flow sensors have limiteddynamic range. For example, a sensor which is accurate at medium flowrates will give inaccurate performance at high or very low flows. Inexisting equipment, it is standard practice to replace the restrictorsto change the range. This requires opening the line while the change isbeing made, and recalibration is usually necessary. Normal practice isto provide factory restrictors which are not changed in the field.

[0051] With the present invention, multiple flow paths can be fabricatedin very small spaces so that the appropriate sensor/restrictorcombination can be used for a desired flow range. This enables havingseparate valves for each flow range in a small package. The circuit ofFIG. 16 has separate flow channel in limited space, thereby enabling aflow controller with much greater dynamic range and hence increasedversatility without increasing cost. One micromachined flow controllermay replace a series of conventional separate flow controllers.

[0052] The circuit of FIG. 16 is controlled by the micropocessorsoftware to determine from the preestablished set-point which of the twochannel paths to open. A flow sensor feeds information back forproportional control. As flow increases, the upper limit of flow isreached, and a separate valve-sensor combination is opened. This systemcan be made without increasing the overall size significantly becausemost of the volume of the controller is the package; sensors and valvescan be orders of magnitude smaller when they are integrated in such apackage.

[0053] The overall control circuit (FIG. 4) reads output from eachsensor and gives feedback so that flow will remain at the desiredsetpoint. This control circuit has certain desirable features: a dataacquisition port monitoring exciter currents, sensor outputs, amplifieroutputs, actuator drivers and currents, and a single connector toconnect the actuator/flow-sensor/press-sensor/temp-sensor package. Thiscircuit also has an actuator-driver that can be scaled to drive up tofour actuators, the selection of which is controlled by the program'soperator screen. Besides being able to logically enable/disable thisdriver, its power is limited so that it should not be able to bum-up anactuator. This circuit also has an LED indicator to show that thePIC-chip has initialized and is ready: if the PIC-chip should be causedto reset, the LED will blink “on” during each initialization routine.

1. A miniature flow controller for use in microelectromechanicalsystems, the flow controller comprising a substrate, the substrate beingformed with a channel for confining a fluid flow, a thin film microvalvemicromachined on the substrate, the microvalve comprising a valveactuator, the actuator having an operating element comprised of a shapememory alloy which undergoes a crystalline phase transformation andresulting shape change from a low temperature deformable phase to a hightemperature memory phase when the element is heated through the alloy'sphase change transformation temperature, the element being positionedfor movement in the channel for contolling the fluid flow responsive tothe shape change, a sensor micromachined on the substrate for sensingfluid conditions in the channel, the sensor being selected from thegroup consisting of a fluid flow sensor, a fluid temperature sensor anda fluid pressure sensor.
 2. A flow controller as in claim 1 and furthercomprising a control circuit which generates a feedback signalresponsive to sensing of the conditions in the channel, the controlcircuit controlling the actuation of the element responsive to thefeedback signal
 3. A flow controller as in claim 1 in which the fluidflow rate sensor is operatively positioned to sense the flow rate offluid in the channel, and the control circuit contols heating of theshape memory alloy responsive to the feedback signal sufficient to causethe movement of the element for adjusting the flow rate in the channel.4. A flow controller as in claim 3 in which the control circuit furtherestablishes a preset flow value and controls the heating of the shapememory alloy sufficient to adjust the fluid flow in the channel towardthe preset flow value.
 5. A flow controller as in claim 3 in which thecontrol circuit controls heating of the shape memory alloy sufficient tocause the element to proportionally adjust fluid flow in the channelwithin a range of flow values.
 6. A flow controller as in claim 1 inwhich the fluid temperature sensor is operatively positioned to sensetemperature of fluid in the channel.
 7. A flow controller as in claim 6in which the control circuit contols heating of the shape memory alloyresponsive to the feedback signal sufficient to cause the element tovary the fluid flow for adjusting the fluid temperature in the channel.8. A flow controller as in claim 1 in which the fluid pressure sensor isoperatively positioned to sense the pressure of fluid in the channel. 9.A flow controller as in claim 8 in which the control circuit contolsheating of the shape memory alloy responsive to the feedback signalsufficient to cause the element to vary the fluid flow for adjustingfluid pressure in the channel.
 10. A flow controller as in claim 1 forproviding multi-channel flow control in which a plurality of thechannels are formed in the substrate, and at least one said microvalveis operatively connected with respective ones of the channels.